To assess the potential health effects of chemically contaminated groundwater, we initiated a toxicological program on a mixture of 25 frequently-detected groundwater contaminants derived from hazardous waste disposal sites. As part of this program, myelotoxicity studies were conducted. Bone marrow parameters were examined in mice exposed to 0, 1, 5 or 10% of a simulated chemical mixture stock solution of groundwater contaminants in drinking water for 108 days. Mice treated with 5 or 10% of chemical mixture stock solution for 108 days showed suppressed marrow granulocyte macrophage progenitors (CFU-GM); however, this suppression disappeared in 10 weeks following the cessation of treatment. The possible toxicological interaction of groundwater contaminants and radiation on hematopoiesis was investigated by using the number of bone marrow CFU-GM as an index. When mice were exposed to 200 rads whole body irradiation at 2 and 9 weeks during this 10-week recovery period, the combined treatment (i.e. , chemical mixture followed by irradiation) group showed a significantly slower recovery of bone marrow progenitors as compared with the control group (i.e., radiation but without prior chemical mixture treatment). This study showed that even 10 weeks after the cessation of chemical mixture treatment when all hematological parameters were normal, a residual effect of the chemical mixture may still be demonstrated as lower progenitor cell numbers following irradiation. Thus, residual damage of hematopoiesis in mice exposed to groundwater contaminants for 108 days renders the mice more sensitive to subsequent irradiation-induced injury. (These results were published in Toxicology Letters, 1991. In Press.)